Special modes-apomixis, parthenocarpy, polyembryony

Introduction

Apomixis is a type of asexual reproduction in plants that involves the development of embryos and seeds rather than the fertilisation of sexual gametes.In the plant kingdom, this is a unique pathway for seed development because, in general, only gamete fusion leads to zygote formation, which leads to embryo formation, and then seed/s formation. In apomixis, however, seed formation happens without fertilisation.

Diplospory, apospory, and adventitious embryony are the three kinds of apomixis that are commonly recognised.

• Diplospory -The embryo sac is generated from the megaspore mother cell through mitotic division or by halting meiosis in diplospory.The megaspore mother cell divides mitotically three times to create eight nuclei in the mitotic diplospory. The embryo sac is not diminished, and the amount of chromosomes and genetic material is the same.
• Apospory-The nucellar cells give rise to the apomictic embryo sac in apospory.In higher plants, it is the most prevalent kind of apomixis. To generate an embryo sac, Aposporos initial cells differentiate and undergo mitosis.It’s not uncommon to find numerous embryo sacs. The formation of the apospory embryo sac and the sexual embryo sac can happen at the same time or it can displace or stop the formation of the sexual embryo sac.
• Adventitious embryony- Embryos are formed straight from the nucellus or integument of the ovule. This is a type of sporophytic apomixis.The embryo grows into a bud-like structure through mitotic division.To create viable seeds, simultaneous fertilisation in the adjacent sexual embryo sac is required. The embryo sac attracts developing embryos in order to obtain nutrients and messages.

There are a few more varieties of apomixis, which are briefly covered below.

• Recurrent and Non-recurrent Apomixis

Both the egg-cell and the embryo are diploid in recurrent apomixis, and the embryos develop from the megaspore mother cell.

Both the egg-cell and the embryo are haploid in non-recurrent apomixis, and the embryo develops directly from an egg-cell without fertilisation.

• Vegetative Apomixis

Instead of flowers, vegetative bulbils or buds are generated in the inflorescence in this apomixis type. They are easily reproducible and can be found in plants such as Fragaria, Agave, and Poa bulbosa.

Parthenocarpy

The production of fruits without the use of ovules is known as parthenocarpy. Fruits such as bananas and figs are grown without fertilisers and yield no viable seeds.Parthenocarpy is a condition in which fruits develop without seeds forming. In the year 1902, this method of fruit production was introduced.This is mostly owing to the lack of fertilisation, pollination, and embryo development in plants. Parthenocarpic fruit is a botanical term that signifies “virgin fruit.” The majority of these fruits are seedless.Parthenocarpy is introduced along with other plant hormones, such as gibberellic acid, during cultivation. It causes the ovaries to mature without the need for fertilisation, resulting in larger, pulpier fruits.Parthenocarpy can be seen in bananas.

Types of Parthenocarpy

Vegetative parthenocarpy and Stimulative parthenocarpy are the two types of parthenocarpy.

• Vegetative parthenocarpy- This usually occurs without pollination, and no seeds are generated within the fruits as a result of the lack of pollination.
• Stimulative parthenocarpy- In most cases, stimulating parthenocarpy occurs without the need for fertilisation. This situation happens when a wasp’s ovipositor is introduced into a flower’s ovary, and it can also be created by blowing air or injecting plant growth stimulants into the syconium’s unisexual blossoms.

Polyembryony

Polyembryony is a phenomenon that occurs when two or more embryos grow from a single fertilised egg.In humans, this results in the formation of two identical twins. This phenomena can be observed in both plants and animals.The nine-banded armadillo is the best example of Polyembryony in the animal kingdom. It’s a medium-sized animal found in areas of America that gives birth to identical quadruplets in the wild.This occurrence occurs in plants as a result of the fertilisation of one or more embryonic sacs or the genesis of embryos outside of the embryonic sac.Antonie van Leeuwenhoek found this natural phenomena in Citrus plant seeds in the year 1719.

Types of Polyembryony 

Polyembryony can be divided into two types:

• Induced polyembryony- Polyembryony that has been induced experimentally
• Spontaneous Polyembryony – naturally occurring

Polyembryony can be divided into three categories, according to Webber:

• Cleavage Polyembryony:In this situation, a single fertilised egg produces a large number of embryos.
• Simple polyembryony: Numerous embryos develop as a result of the fertilisation of several archegonia in this kind.
• Rosette polyembryony:Rosette polyembryony is a kind of polyembryony in which additional embryos emerge from rosette cells in certain gymnosperms.

Apomixis

An asexual reproduction that occurs without fertilisation but results in the production of embryo(s) and seed is known as apomixis. Parthenogenesis, which occurs when an egg cell develops directly into an embryo without the need for prior fertilisation, is one example of apomictic evolution. As a result, the offspring produced by apomictic parthenogenesis would be complete clones of their mother.

The process of reproduction is critical to the life cycle of plants, just as it is to the life cycle of any other organism. Various mechanisms exist in plants for carrying out the necessary reproductive process. Here are a few examples. All of them fall under one of two broad categories of reproduction: sexual reproduction and asexual reproduction. Within a sexually reproduced individual, two gametes of different genders fuse and result in the process of fertilisation, which results in the production of a progeny or progenies with a newly formed genetic make-up. In contrast, there is no gamete fusion or fertilisation step in asexual reproduction, and the progeny produced is genetically identical to the parental generation in both cases.

Conclusion

Apomixis (asexual seed creation) is a phenomena in which a plant forms a viable seed without going through the most basic parts of sexual reproduction—meiosis and fertilisation.Plants can produce seeds without fertilisation, and the genotype of the seed is identical to that of the female parent.The development of apomictic technology would be revolutionary for agriculture and food production because it would cut expenses and breeding periods while also avoiding many of the problems associated with sexual reproduction (for example, incompatibility barriers) and vegetative propagation (e.g. viral transfer).

Early fruit production and harvest are possible thanks to parthenocarpy. As a result, parthenocarpy can be used to rationalise and increase fruit quality and yield in plant species that are grown for their fruits.

Polyembryony is a type of asexual reproduction in which a single egg or zygote produces numerous children.Polyembryony, also known as embryonic cloning, is the division of a single sexually generated embryo into multiple offspring who are genetically identical to each other but separate from their parent(s), asexual budding being the opposite.